This is the "back door" into the IO monad, allowing
IO computation to be performed at any time. For
this to be safe, the IO computation should be
free of side effects and independent of its environment.

If the I/O computation wrapped in unsafePerformIO performs side
effects, then the relative order in which those side effects take
place (relative to the main I/O trunk, or other calls to
unsafePerformIO) is indeterminate. Furthermore, when using
unsafePerformIO to cause side-effects, you should take the following
precautions to ensure the side effects are performed as many times as
you expect them to be. Note that these precautions are necessary for
GHC, but may not be sufficient, and other compilers may require
different precautions:

Use {-# NOINLINE foo #-} as a pragma on any function foo
that calls unsafePerformIO. If the call is inlined,
the I/O may be performed more than once.

Use the compiler flag -fno-cse to prevent common sub-expression
elimination being performed on the module, which might combine
two side effects that were meant to be separate. A good example
is using multiple global variables (like test in the example below).

Make sure that the either you switch off let-floating (-fno-full-laziness), or that the
call to unsafePerformIO cannot float outside a lambda. For example,
if you say:
f x = unsafePerformIO (newIORef [])
you may get only one reference cell shared between all calls to f.
Better would be
f x = unsafePerformIO (newIORef [x])
because now it can't float outside the lambda.

This program will core dump. This problem with polymorphic references
is well known in the ML community, and does not arise with normal
monadic use of references. There is no easy way to make it impossible
once you use unsafePerformIO. Indeed, it is
possible to write coerce :: a -> b with the
help of unsafePerformIO. So be careful!

This version of unsafePerformIO is more efficient
because it omits the check that the IO is only being performed by a
single thread. Hence, when you use unsafeDupablePerformIO,
there is a possibility that the IO action may be performed multiple
times (on a multiprocessor), and you should therefore ensure that
it gives the same results each time.

unsafeInterleaveIO allows IO computation to be deferred lazily.
When passed a value of type IO a, the IO will only be performed
when the value of the a is demanded. This is used to implement lazy
file reading, see hGetContents.